Decorative laminating papers and decorative foils are part of a decorative, thermosetting coating material, used with preference for finishing furniture surfaces, for laminate flooring and in interior finishing. The term “laminates” is used to denote materials in which, for example, several impregnated, stacked layers of paper, or paper and hardboard or fiberboard, are pressed together. The use of special synthetic resins achieves extraordinarily high resistance of the laminates to marring, impact, chemicals and heat.
The use of decorative laminates makes it possible to produce decorative surfaces, where the decorative laminating paper serves not only as facing paper, e.g. to hide unattractive wood material surfaces, but also as a carrier for the synthetic resin. The demands imposed on decorative laminating paper include, among others, opacity (hiding power), light-fastness (greying resistance), color-fastness, wet strength, suitability for impregnation and printability.
The economic efficiency of the manufacturing process for decorative laminating papers is determined by, among other things, the opacity of the pigment in the paper. In principle, a pigment based on titanium dioxide is eminently suitable for achieving the necessary opacity of the decorative laminating paper. During paper manufacture, a titanium dioxide pigment, or a titanium dioxide pigment suspension, is usually mixed with a pulp suspension. In addition to pigment and pulp as the feedstock, use is generally also made of auxiliaries, such as wet-strength agents, and further additives where appropriate. The interactions of the individual components (pulp, pigment, auxiliaries and additives, water) with each other contribute to formation of the paper and determine the retention of the pigment. Retention is the capacity for retaining all inorganic substances in the paper during production.
It is known that an improvement in opacity can be achieved by special treatment of the surface of the titanium dioxide pigment.
U.S. Pat. No. 5,942,281 A and U.S. Pat. No. 5,665,466 A describe a surface treatment in which a first layer of aluminium oxide phosphate is applied at an acidic pH value of 4 to 6, and a second layer of aluminium oxide is precipitated in a pH range from 3 to 10, preferably at roughly pH 7. An improvement in retention is achieved by a third layer consisting of magnesium oxide, the result being that the pigment produced is characterized by consecutive layers of aluminium oxide phosphate, aluminium oxide and magnesium oxide.
U.S. Pat. No. 6,962,622 B2 discloses a titanium dioxide pigment blend that is composed of a pigment with high greying resistance (pigment Type A) and a pigment with a coating displaying an elevated SiO2 and Al2O3 content, precipitated in fluffy form (pigment Type B).
U.S. Pat. No. 6,143,064 A describes the coating of pigment particles with precipitated calcium carbonate, where the size of the calcium carbonate particles is between 30 and 100 nm. The titanium dioxide coated with calcium carbonate achieves higher opacity in the paper. In this context, the calcium particles assume the function of spacers, such that the pigment particles display better distribution in the paper. The minimum distance between the pigment particles is said to correspond roughly to the size of the pigment particles.
U.S. Pat. No. 5,886,069 A and U.S. Pat. No. 5,650,002 A describe TiO2 pigment particles displaying both a continuous inorganic coating and a coating of discrete inorganic particles with a diameter of 5 to 100 nm. The surface coating sequence can be selected at random, as can the form of the discrete particles. Production is performed by mixing a colloidal suspension of the discrete particles with a TiO2 slurry.
U.S. Publication 2003 0024437 A1 discloses a pigment blend with pigment particles onto whose surface solid spherical particles, such as calcium carbonate, silicon oxide, aluminium oxide, zirconium oxide or titanium oxide, have been precipitated in-situ.
A need has arisen for an alternative titanium dioxide pigment with good opacity and good retention for use in decorative laminating papers. A need has further arisen for a method for manufacturing a titanium dioxide pigment of this kind.